1. Technical Field
The present disclosure relates to the field of vertebral disc implants, i.e., disc implants having applicability throughout the vertebral column. Exemplary embodiments of the disclosed vertebral disc implants advantageously and ultimately provide fusion with the body of the vertebra and stabilization of the spine (e.g., cervical, thoracic and/or lumbar spinal regions) in an anatomically correct position. More particularly, the present disclosure is directed to a disc implant that addresses and overcomes the shortcomings of the prior implants by providing first and second inter-vertebral elements that are movably coupled relative to each other and are adapted to permit bone in-growth over time. Thus, the disclosed spinal disc implants permit relative movement between the first and second inter-vertebral elements upon implantation and after the patient is mobilized—thereby permitting the implant to assume a desired position based on the specific and unique spinal balance of the patient in an initial post-implantation period—but then the first and second inter-vertebral elements become fixed relative to each other (i.e., fused). The present disclosure also provides advantageous instrumentation and associated methods for positioning a spine disc implant in a desired anatomical location.
2. Background Art
Back pain with or without leg pain is a major problem in the adult population. The pain may have multiple causes and in certain instances, surgery may be required to mitigate such pain. Lower back pain may be caused by displacement of vertebrate bodies and/or intermediate discs in the lumbar region of the spine. The L4-L5 and L5-S1 regions of the spine are particularly vulnerable. For patients with severe pain that doesn't respond to conservative treatment, fusion surgery is currently viewed as a viable option. Spinal fusion surgery (fusing one vertebra to another) is often performed to decrease motion at a painful motion segment, thereby reducing associated pain at that segment. This abnormal and painful motion phenomenon can be caused by disc-related issues (e.g., discogenic pain and/or degenerative disc disease), abnormal slippage and motion of the vertebra (e.g., spondylolisthesis and/or spondylolysis), or other degenerative spinal conditions, including but not limited to facet joint degeneration. In addition, a spine fusion may be indicated for any condition that causes excessive instability of the spine, such as certain fractures, infections, tumors and/or spinal deformity (such as scoliosis).
Interbody surgeries may be performed either from the front or from the back of the patient and exemplary procedures are described as “posterior lumbar interbody fusion” (PLIF), “transforaminal lumbar interbody fusion” (TLIF), “lateral lumbar interbody fusion” (XLIF), “anterior interbody fusion” (AIF), “anterior lumbar interbody fusion” (ALIF) and “anterior cervical discectomy and fusion” (ACDF). As a group, the noted fusion procedures are generally referred to as “circumferential fusion.” Each of the noted circumferential fusion procedures generally involves removing a disc from between two adjacent vertebrae and inserting a structure, e.g., bone, into the space created between the two vertebral bodies. During conventional posterolateral spine fusion (PLF) surgery, a graft is generally laid out in the posterolateral portion of the spine. Posterior surgery has been shown to yield acceptable clinical results and is claimed to further improve outcome by adding anterior column support. However, posterior surgery procedures are unfortunately associated with a long recovery compared to exclusively anterior surgery.
In general, the position of the disc implant is determined during surgery. The position is influenced by factors such as the manner of fixation employed by the surgeon and/or by the design of the implant used. As the fusion requires stabilization until bone growth has occurred, which may often take several months (e.g., 3-6 months), the position is important to achieving a fusion. If the position is not correct, the surgery may result in a non-union (a failure to achieve fusion) or may even result in secondary effects caused by stress placed on the neighboring discs. If necessary, subsequent surgeries are complicated by the previous surgery.
Three general types of “total disc replacement” (TDR) implants are known. A first TDR implant may be characterized as an unconstrained design and such design appears to have some advantages as unconstrained designs are more likely to provide a physiologic mobile instantaneous axis of rotation (IAR), thus displaying a greater range of motion in vivo. The lack of constraint in such unconstrained designs may prevent excessive facet joint or capsuloligamentous loads in the extremes of flexion and extension. Furthermore, since the IAR is mobile, the unconstrained designs may be less sensitive to small errors in implant placement, for example, the Charite total disc replacement. A second TDR implant may be characterized as a constrained design and constrained devices appear to have an advantage in protection of the posterior elements from shear loading, for example the FLEXICORE implant (Stryker Spine, Allendale, N.J.). Spinal shear loads of considerable magnitude occur during activities of daily living. A third group of TDR implants may be characterized as semi constrained implants and include commercially available products such as the PRODISC implant (Synthes Spine, West Chester, Pa.).
The patent literature reflects efforts to address issues related to back and/or leg pain and/or spinal instabilities. Thus, for example, U.S. Patent Publication No. 2006/0235529 to Ralph et al. (the “Ralph '529 publication”) is directed to a disc implant that is expressly designed to ensure that fusion does not occur. The Ralph implant features opposed first and second plates that are movably coupled relative to each other. Exemplary embodiments of the Ralph '529 publication include a “spider spring” that exhibits “long cycle life to mimic the axial biomechanical performance of the normal human intervertebral disc.” [Ralph '529 publication, para. 0030] The exterior surfaces of the Ralph implant include a convex mesh and a porous ring that facilitate bone in-growth so as to anatomically fix the implant, i.e., “permanently securing the prosthesis within the invertebral space.” [Ralph '529 publication; para. 0019] Thus, the Ralph '529 publication contemplates bone in-growth to fix the first plate relative to a first vertebral body and bone in-growth to fix the second plate relative to a second vertebral body, while maintaining relative motion between the first and second plates. The Ralph '529 publication is thus clearly designed to ensure that bone in-growth between the first and second plates is avoided. The Ralph '529 publication explicitly identifies conventional fusion cage technology as inferior because, according to Ralph et al., bone fusion is both undesirable and detrimental to patients.
U.S. Pat. No. 6,641,614 to Wagner et al. (the “Wagner '614 patent”) provides a fusion device that includes a pair of engaging plates and an alignment device positioned therebetween. The alignment device is adapted to adjust the height between the engaging plates, thereby permitting customization of the fusion device to a particular patient. Of note, the height of the Wagner fusion device is generally adapted to differ from anterior end to posterior end. The alignment device of the Wagner '614 patent necessarily relies on the surgeon's judgment in arriving at an “adjusted” relative position between the engaging plates. In exercising such judgment, the surgeon is required to interact with the adjustment device using specialized instrumentation. Thus, the “adjustability” enabled by the Wagner fusion device is, at best, dependent on a surgeon's clinical experience in arriving at an “adjusted” height between engaging plates that may—or may not—be advantageous for the specific patient. Indeed, method(s) for reliably selecting the correct spinal position on an individual basis at the time of surgery is/are not currently available.
As with conventional fusion devices, the engaging plates of the Wagner '614 patent include “a plurality of openings to allow bone growth to occur through the engaging plates,” e.g., openings having a total area of about 60% to 80% of the total surface area of the engaging plates. [Wagner '614 patent, col. 2, lines 37-42] Once implanted and height-adjusted, the Wagner fusion device is fixed in position relative to the adjacent vertebrae. Thus, unlike the disc implant of the Ralph '529 publication, the Wagner fusion device is not adapted and/or intended “to mimic the functionality of a healthy natural invertebral disc.” [Ralph '529 publication, para. 0118]
Thus, as the Ralph '529 publication and the Wagner '614 patent illustrate, the teachings in the patent literature have generally fallen into two (2) distinct categories: a first category represented by the Ralph '529 publication that are designed to ensure continued relative movement between opposed plates post-implantation, and a second category represented by the Wagner '614 patent that are designed to effectuate prompt fusion between adjacent vertebrae post-implantation.
Against this backdrop, an innovative approach to spinal stabilization is disclosed in U.S. Pat. No. 8,007,536 to Christensen which discloses a disc implant that includes two inter-vertebral elements which are flexibly connected via coupling means. Following surgery, the relative movability of the two inter-vertebral elements is decreased overtime, as bone in-growth occurring around the implant and specifically through osseointegrative sections gradually decrease the movability of the elements relative to each other. Following bone in-growth, the relative movability of the implant elements is replaced by relative fixation of the elements. The fixation of the spinal implant advantageously occurs in a position affected by the movement of the patient and the loading in the patient's spine, and is thereby more acceptable to the patient. The entire content of the Christensen '536 patent is incorporated herein by reference.
A goal of the present disclosure is to further improve the previous work of Christensen in enabling fixation of a spinal region, e.g., cervical, thoracic and/or lumbar spinal region, in a position affected by the movement of the patient and the loading in the patient's spine. In addition, the present disclosure provides instrumentation and implant(s) that permit/facilitate placement of an implant in a desired orientation/position, e.g., through cooperative features. These and other needs are satisfied by the spinal implants disclosed herein.